It has long been conventional to measure changes in altitude by means of mechanical instruments sensitive to changes in pressure of the earth's atmosphere from one elevation to another. Accordingly, it is conventional to provide an aircraft with one or more static pressure ports so that the external air pressure is exerted upon a pressure measuring diaphragm containing within the aircraft. However, significant inaccuracies may result from disturbances of the airflow in the region of the pressure port caused by icing, by air currents and turbulence, and by air compression effects or from changes in the orientation of the port relative to the airflow caused by changes in the attitude of the aircraft.
Since it is a well known law of nature that the pressure of a gas is linearly related to its density and temperature, it is also possible to compute the pressure of the air at a given elevation from measurements of other physical quantities of the air such as air density and temperature, and then to use the thus computed pressure to determine barometric altitude.
U.S. Pat. No. 2,925,007 in the name of Marvin N. Silver teaches a method and device for measuring the pressure of a gas inside a transparent vessel, such as a vacuum tube in a laboratory. Silver assumes that the amount of light that will be scattered at a given acute angle with respect to the forward direction of the light propagation is proportional to the amount of gas contained within the vessel and accordingly employs a photodetector to measure the amount of light scattered forwards from a beam of light rays projected into the enclosure. However, a simple scattered light type of device would not be suitable for measuring the air pressure external to a flying aircraft, since the light-detector would be exposed not only to the scattered light originating from the projected beam of light rays, but also to unpredictable quantities of scattered and direct background radiation from the sun and other celestial objects. Furthermore, in the laboratory the composition of the gas remains constant while the vessel is being evacuated; accordingly, it is immaterial whether or not the gas contains an exceptionally large proportion of aerosol particles which will cause a significantly greater percentage of the projected light to be scattered in the direction of the photodetector. Obviously, any measurement technique that assumes that the composition of the earth's atmosphere is homogeneous would be subject to significant errors. Finally, if both the light emitter and the light detector are to be contained within a single compact unit, then it becomes impractical to employ any forward scattering technique since the detection volume cannot then be located at a sufficiently remote distance from the aircraft that it is in the free air stream, and not subject to disturbance by the aircraft itself.
U.S. Pat. No. 4,071,298 in the name of David G. Falconer teaches the use of various techniques to detect and measure individual, relatively large aerosol particles, particularly cyclic aromatic hydrocarbons and aldehydic and ketonic derivatives. Among the techniques discussed are the use of a laser to cause the target molecules to fluoresce and the use of a narrow band filter to pass only the frequencies of interest. By such means, Falconer is able to measure light received from a single aerosol particle and thereby to determine the effective cross-sectional area of the scattering particles. Although the aerosol particles being analyzed are contained within a stream of air, it is to be emphasized that Falconer's system is insensitive to any physical parameters such as pressure or density associated with the air stream itself.
U.S. Pat. No. 4,099,872 in the name of John U. White teaches the use of a fluorescent spectrophotometer in which a beam of radiation from a xenon arc ("or other suitable source of visible or invisible light") is directed by means of a suitable optical system onto a sample and causes the sample to emit fluorsecence of a wavelength different from that of the excitation light source. White's system utilizes monochrometers for selecting a highly monochromatic portion of the luminescent emission from the sample and focuses this monochromatic portion on a photo-electric detector which produces an output signal proportional to the intensity of the light. By means of a rotating beam chopper that alternatively interrupts the excitation beam impinging on the sample and a reference beam also originating from the same excitation source, the detector is alternatively illuminated by the fluorescent emission and by the excitation beam, thereby resulting in an output signal level which alternately represents the unknown luminescent intensity from the sample and the intensity of the reference beam whereby a signal may be generated corresponding to the ratio of the net sample signal to the net reference signal.
U.S. Pat. No. 3,850,525 in the name of Wilbur I. Kaye teaches the employment of two different radiant energy detectors in a single system, whereby scattered light from a solid or liquid sample in a laser light scattering photometer may be the subject of simultaneous multiple measurements (e.g., at two different angles or at two different wavelengths). Such a system is sensitive to background radiation and Kaye attempts to minimize any undesired increase in the background level caused by the diffraction from the inner edge of an annulus used to define the solid angle of interest by providing a mirror to transmit the diffracting rays to a light trap rather than to the detector.
U.S. Pat. No. 3,958,108 in the name of Naomobu Shimomura relates generally to barometric altimeters such as those used in aircraft and more specifically to electronic barometric altimeters in which an electric signal from a mechanical pressure transducer is conditioned so as to provide an accurate digital indication of aircraft altitude in accordance with well known air data equations.
It has also been proposed to use a high power pulse laser transmitter and a receiver with range gating circuitry to look at the fluorescence or Raman scattering return signal from a localized region of the atmosphere and thus to determine concentrations of various molecular constituents in such region and, in particular, the concentration of pollutants such as NO, NO.sub.2, CO, SO.sub.2, and O.sub.3 (nitrous oxide, nitric oxide, carbon monoxide, sulphur dioxide, and ozone). For some applications, knowledge of the local concentration of H.sub.2 O and/or CO.sub.2 (water and/or carbon dioxide) may also be desirable. With regard to the foregoing, an article appearing in Optical and Quantum Electronics for 1975 at pages 147-177 entitled "Review: Remote Air Pollution Measurement" by R. L. Byer and an article entitled "A Traveler Returns" (concerning a specially equipped aircraft in use by the National Oceanic and Atmospheric Administration) appearing at page 9 of Patrol Log for Fall 1981 (published by the assignee of the present invention) are additionally cited as exemplary of various prior art approaches to air data measurements.
However, taken as a whole, the known prior art does not teach or suggest how air data parameters that are critical to the operation of an aircraft (such as density, pressure, and/or barometric altitude) can be simply and reliably measured by means of an accurate and reliable measurement device free of any pressure ports or protrusions into the airstream.
Furthermore, such known prior art does not teach or suggest any reliable method for making air data parameter measurements at a sample location at a sufficient distance from the aircraft or any physical attachments thereto that the measurement will not be subject to systemic errors of a sort that cannot always be fully compensated for such as those caused by air compression effects and airflow disturbances.
Moreover, the known prior art does not teach or show how fluorescent emissions, and, in particular, how the decay characteristics of such fluorescent emissions once the fluorescent energy source has been interrupted, may be utilized to calculate air data parameters such as the relative density of a particular molecular species or, if the fluorescing molecules represent a known percentage of the atmosphere, the density, pressure and barometric altitude of the atmosphere at the elevation at which the measurement is being made.
The teachings and disclosures contained in the above-referenced U.S. patents and the information provided by the other above-referenced publications may contribute to a better understanding of the background of the present invention, as well as of its scope, function and possible manners of implementation and use; accordingly, they are hereby incorporated in their entirety by reference the same as if fully set out herein.
Accordingly, it is one object of the present invention to use fluorescence to sense air data parameters.
It is a related object to provide a system which is capable of examining by optical means contained within an aircraft physical properties of an air sample in a nearby free airstream to determine aircraft altitude, air density, water vapor content, carbon dioxide content, and the like.
Another related objective is to provide a system of the type described that may have its sensor unit mounted inside the unbroken aerodynamic skin of the aircraft so as to result in no drag penalty.
Yet another related objective is to provide an aircraft flight performance measurement system that may be integrated with a radiation velocimeter so as to completely dispense with the need of any conventional pneumatic systems such as Pitot static probes, pneumatic tubing, and pressure transducers.
It is yet another related objective to provide such a system wherein the sensor components could be mounted flush with the skin of the aircraft.
It is a more specific object to provide a fluorescence altitude measuring system for optically measuring the density of an air mass and/or of one or more principal constituents thereof located at some distance from the aircraft where the air mass is relatively undisturbed.
It is yet another more specific object of the invention to provide such an altitude measurement system wherein the physical interface between the sensor unit inside the aircraft and the external air mass whose properties are being measured is through a window mounted flush with the skin of the aircraft.
Another more specific object is to cause intermittent fluorescence of selected air molecules within a sample volume located external to the aircraft, to observe optically the decay constant associated with such fluoresced air molecules, and then to employ those observations by solving basic air data equations to result in calculated aircraft flight parameters such as air density, barometric altitude, and density altitude, as well as the concentration of said selected molecules relative to the balance of the sample.
An overall object of the present invention is to provide a system for measuring barometric altitude or other density-related parameters by utilizing an electro-optical system mounted inside an aircraft to determine the physical properties of an external air mass located at a distance from the aircraft.